Contamination control, including particulate monitoring, plays a critical role in the manufacturing processes of several industries. These industries require cleanrooms or clean zones with active air filtration and require the supply of clean raw materials such as process gases, de-ionized water, chemicals, and substrates. For example, the Food and Drug Administration requires the pharmaceutical industry to monitor particulates because viable particles that contaminate products are closely correlated to detected particles in an aseptic environment. Semiconductor fabrication companies also require particulate monitoring as an active part of quality control. As integrated circuits become more compact, line widths decrease, and the sizes of particulates that cause quality problems become smaller.
Conventional optical particle detection relies on the direct detection of Rayleigh scattering of light by the particles (for particles that are small compared to the wavelength). Rayleigh light scattering intensity (I.sub.sc) equals I.sub.o k/.lambda..sup.4, where I.sub.o represents an intensity of incident output radiation and .lambda. represents a wavelength of the incident output radiation. Particle size information is determined from the k coefficient based on the detected scattered intensity. Because light scattered by submicron particles is of small intensity, high incident intensity is necessary to achieve detectability. Therefore, to improve I.sub.sc measurements, the incident light intensity is preferably maximized by employing high intensity laser light. Because light intensity is higher inside a closed laser cavity, the incident light intensity is further increased by detecting intracavity light scattering. Unfortunately, optically pumped lasers conventionally used for particle detection generate an output wavelength that is longer than its pumping wavelength. This reduces the detectable Rayleigh scattering due to the inverse fourth power dependence of scattering intensity on the laser wavelength.
Although the wavelength from a "normal laser" can be converted in nonlinear crystal by harmonic generation, the efficiency of nonlinear conversion is limited. The efficiency of second harmonic generation depends on the intensity at the fundamental wavelength. Single pass conversion efficiency is typically far below 1% in the case of cw lasers of low intensity. Some applications (for example, biological particle characterization based on the measurement of protein autofluorescence) utilize UV light. Two nonlinear crystals have to be employed to generate the UV light whenever a conventional "normal" diode-pumped solid-state laser is the source of fundamental radiation. This results in even lower efficiency of nonlinear conversion. The employment of two nonlinear crystals also makes the laser system more complicated from a manufacturing and reliability standpoint.
Q-switched laser systems have been used for harmonic generation and subsequent particle characterization as taught by R. G. Pinnick, S. C. Hill, P. Nachman, G. Videen, G. Chen, R. K. Chang, Aerosol Science & Technology, v. 28, p. 95-104 (1998). Pulsed laser systems provide an easy technical solution for efficient generation of UV light in nonlinear crystals due to high peak power achievable in such lasers. Unfortunately, particles can be missed if traveling through the view volume between pulses. Therefore cw laser operation is preferred for particle counting.